Use of nitrogen-filled ion chamber for wide-range gamma monitoring

A gamma-sensitive ion chamber has been developed for area gamma monitoring. it has 800 cc sensitive volume tilled with nitrogen at 120 psig pressure. With a current sensitivity of 0.6 nA/R/h the chamber needs 250 V for 90 % s aturation at 10R/h. However, the use of the tanh formula proposed by Fallon e and Podgorsak (Med Phys. 10 (1983) 191) has enabled the authors to develo p an instrument to derive exposure rates as high as 80,000 R/h with the hel p of a single current reading at an applied voltage of 300 V. The instrumen t can be used to measure unknown exposure rate with accuracy within +/- 3.5 %.